Electronic tube



Aug. 5, 1941. c. A. MORTON ELECTRONIC TUBE Filed May 15, 1937 PIC Tumsou'l'Pu'r W ELECTRON MUL '7'IPL [ER 2 Sheets-Sheet l 3maentor (Ge urgeH. Morton Bu Ctttorngg.

1941- G. A. MORTON 2,251,573

' ELECTRONIC TUBE Filed May 15, 1937 2 Sheets-Sheet 2 Snventor GeorgeH.Mo rt0n (Ittorneg Patented Aug. 5, 1941 UNIT STATS AT FFICE ELECTRONICTUBE Delaware Application May 15, 1937, Serial No. 142,748

9 Claims. (Cl. 259-450) My invention relates to electronic tubes andparticularly to cathode ray tubes of the type employed for televisiontransmission.

Specifically, my invention relates to electronic tubes of the typedescribed and claimed in Flory and Morton Patent No. 2,093,166, issuedSeptember 14, 1937, and assigned to the Radio Corporation of America.Tubes of this type comprise a photoelectric screen upon which isprojected the image of a subject to be transmitted. The resultingelectrical image appearing on the screen is scanned by a cathode raywhereby secondary electrons are released from successive points on thescreen, the secondary electrons released from a particular pointdepending upon the intensity of the light image at that point.

The secondary electrons, as they are released from the screen, arepulled into an electron multiplier which multiplies them and suppliesthem to an output circuit.

An object of my invention is to increase the sensitivity of anelectronic tube of the abovedescribed type.

Broadly speaking, it is an object of my invention to provide an improvedcathode ray transmitter tube of the desired sensitivity for picturetransmission.

A further object of my invention is to increase the picture signal tonoise ratio of a cathode ray transmitter tube.

In the above-described combination of cathode ray transmitter tube andelectron multiplier, the shot noise in the scanning beam imposes a limitto the tube sensitivity. In a tube of conventional design it isimpossible to reduce the beam current, and, thus, the shot noise,because the capacity of each element or globule of the photo-electricscreen is so large that the elements will not reach equilibrium if thebeam current is reduced. By equilibrium is meant the condition where thenumber of primary electrons reaching an element is equal to the numberof secondary electrons leaving it. Therefore, if the beam current isreduced, the signal output is also reduced.

In accordance with my invention, I so construct the photo-electricscreen that the capacity of each photo-electric element is greatlyreduced whereby the beam current may be reduced without decreasing thesignal output. Thus, the shot noise is reduced and the sensitivity ofthe tube increased.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings in which Figure 1 isa view of one embodiment of my invention, and

Figure 2 is a view of one of the preferred embodiments of my invention.

Referring to Fig. 1, the transmitter tube comprises an evacuatedenvelope I having therein an indirectly heated cathode 2, a controlelectrode 3, a first anode 4 and a second anode 6. These electrodesgenerate an electron beam, which is electrostatically focused, anddirect it towards a photo-electric screen 1. The beam is caused to scanthe screen 1 by any well known means such as deflecting coils (notshown).

The screen structure 1 comprises a sheet or plate 8 of insulatingmaterial such as glass or mica which has on the front side thereof amosaic of minute photo-sensitive elements or globules 9 which aresubstantially insulated from each other. The mosaic of elements 9 may beformed in any one of several ways as by ruling a coating of silver whichhas been evaporated upon the supporting sheet 8, and then sensitizingthe ruled silver with an alkali metal such as caesium. Preferably,however, the elements 9 are photo-sensitive silver globules formedthereon by the method described in Patent No. 2,065,570, issued December29, 1936, in the name of Sanford E. Essig, and assigned to the RadioCorporation of America. When formed by this method, the globules aremicroscopic in size.

The screen structure I may be supported in any suitable manner as bymeans of a yoke II. It will be noted that in the specific embodiment ofthe invention illustrated in Fig. 1, the screen structure 1 has noback-plate or metal coating on the back side of the member 8.

In transmitting a picture, an image of the sub ject to be transmitted,indicated at I2, is projected upon the screen 1 whereby each of thephoto-sensitive capacity elements 9 acquires a charge depending upon theintensity of light striking that particular element. Thus an electricimage of the subject I2 is produced on the screen 1.

Now when the screen I is scanned by the electron beam, secondaryelectrons are released from each of the elements 9, the number ofsecondary electrons leaving each element depending upon the charge orpotential of that element. These secondary electrons are pulled into anelectron multiplier indicated at 14 where they are multiplied oramplified and supplied to an output conductor [6.

The electron multiplier is in the same envelope l as the electron gunand photo-electric screen as described in the above-mentioned Morton andFlory patent, and the multiplier structure and connections may be thesame as described in this patent. If preferred, the multiplier may bethe same as that described hereinafter in connection with Fig. 2.

Since the screen I has no back-plate, the capacity of each element 9 isvery small and the intensity of the electron scanning beam may begreatly reduced whereby the shot efiect is reduced and the sensitivityof the tube increased. The word sensitivity is used here in the broadsense that the tubes ability to transmit good pictures when the subjectis not strongly illuminated is improved. Decreasing the photosensitiveelement capacity and at the same time decreasing the beam current doesnot change the signal sensitivity appreciably but it does reduce thenoise without reducing the picture signal current. Therefore, thepicture signal to noise ratio of the signal output of the tube isgreatly improved.

As will be more fully explained in connection with Fig. 2, it is notnecessary that the backplate" be omitted from the screen structure I asthe capacity of the elements may be sufiioiently reduced by making thesupporting plate 8 considerably thicker than the plate employed in atube of more conventional design.

Referring to Fig. 2, one of the preferred embodiments of my invention isillustrated, the screen structure indicated at 20 being shown with athick insulating plate 2| and a back-plate" 22. It will be understoodthat the back-plate 22 may be omitted as shown in Fig. 1, this being thelimiting condition for reducing the capacity of the photo-sensitiveelements.

Except for the screen structure 20 and the adjustment of the electronbeam current, the tube and circuit shown in Fig. 2 are the same asdescribed and claimed in application Serial No. 122,690, filed January28, 1937, and in application Serial No. 128,050, filed February 27,1937, both filed as joint inventions of Leslie E. Flory and George A.Morton, and assigned to the Radio Corporation of America.

In this embodiment of the invention, the picture transmitting tubecomprises an evacuated envelope 23 having therein the mosaic screenstructure 29 which corresponds to the screen 1 in Fig. 1 and which, likescreen I, is set at an angle to permit the projection of a light imagethereon. In order to simplify the drawing, the screen 20 is notillustrated at this angle. The envelope also includes an electron gun 24and two electron multipliers 26 and 21. A second anode which is providedfor accelerating and focusing the electron beam is in the form of acontinuous silver coating on the envelope 23 as indicated at 28, thissilver coating extending around the necks of the electron multipliers asindicated at 28a and 28b.

The electron multipliers 26 and 21, which are so spaced as to collectsecondary electrons uniformly from the screen 20, are identical inconstruction. Referring to the multiplier 21, it comprises a collectorplate 3| and a truncated cone electrode 32 which pull the secondaryelectrons into the multiplier where they strike the T- shaped element33. Secondary electrons are released from the inner surface of element33 and pulled into T-shaped element 34 where more secondary electronsare released. These are pulled into element 36 where they are furthermultiplied. The final output appears on the plate 31 and is supplied tothe input circuit of an amplifier tube 38 where the picture signal isfurther amplified. It will be noted that the elements 3|, 32, 33, etc.,are at successively higher potentials, being supplied with voltage froma voltage divider 40.

As shown, the multiplier 26 is connected in parallel with the multiplier21. Preferably, the collector plates 3| and 3| of the two multipliersare connected to the voltage divider 40 through separate taps wherebytheir voltages may be separately adjusted.

The back-plate 22 and the second anode 28 are connected to the negativeterminal of the voltage divider 40.

Referring more specifically to the screen structure 20, it has on itsfront surface a mosaic of photo-sensitive elements 4| which maybe formedas described in connection with Fig. 1. In this screen structure thecapacity of elements 4| is reduced by making the insulating plate 2|very thick as compared with usual practice. For example, the plate 2|may be 10, 20 or 30 mils thick as compared with a plate 1 mil thick in astandand mosaic screen structure.

The beam current usually should be substantially inversely proportionalto the thickness of the insulating plate 2|. For example, in a standardtube the beam current is about one-half micro-ampere, while the micaplate of the mosaic screen is about 1 mil thick. In the tube shown inFig. 2, however, if the plate 2| is 30 mils thick, the beam currentpreferably is reduced to approximately micro-ampere. Or, if the plate 2|is- 15 mils thick, the beam current is reduced to the order ofmicro-ampere.

In practice, when adjusting one of my improved tubes for transmitting apicture, the electron beam current is reduced to a value such that anyfurther substantial reduction in beam current would impair the qualityof the transmitted picture. In a preferred tube, the capacity of theeffective picture element (that is, the area of the mosaic under theelectron beam) is reduced to a value such that the beam current may bereduced to a value not greater than ten times the photoelectric currentfrom the mosaic to obtain a signal to noise ratio which is at least ten.The beam current should never be less than the photoelectric current.

I claim as my invention:

1. A cathode ray picture transmitting tube comprising an evacuatedenvelope having therein an electron gun for producing an electron beam,a screen positioned to be scanned by said beam, said screen comprising asheet of insulating material having on the front side thereof facingsaid gun a mosaic of secondary electron emissive elements and having onthe back side thereof an electrically conducting sheet or backplate,said elements being insulated from each other and said insulating sheetbeing at least 10 mils thick, and an electron multiplier so located insaid envelope as to receive the secondary electrons which are releasedfrom said mosaic when scanned by said beam, the beam current being ofthe order of from micro-ampere to micro-ampere.

2. The invention according to claim 1 characterized in that saidphoto-electric elements are microscopic photo-sensitized silverglobules.

3. A cathode ray picture transmitting tube comprising an evacuatedenvelope having therein an electron gun for producing an electron beam,a screen positioned to be scanned by said beam, said screen comprising asheet of insulating material having on the front side thereof facingsaid gun a mosaic of elements and having on the back side thereof anelectrically conducting sheet or back-plate, said elements beinginsulated from each other and said insulating sheet having a thicknessof the order of 20 mils, and an electron multiplier so located in saidenvelope as to receive the second electrons which are released from saidmosaic .when scanned by said beam, the beam current being of the orderof from micro-ampere to micro-ampere.

4. The invention according to claim 3 characterized in that saidphoto-electric elements are microscopic photo-sensitized silverglobules.

5. A cathode ray picture transmitting tube comprising an evacuatedenvelope having therein an electron gun for producing an electron beam,a screen positioned to be scanned by said beam, said screen comprising asheet of insulating material having on the front side thereof facingsaid gun a mosaic of elements and having on the back side thereof anelectrically conducting sheet or back-plate, said elements beinginsulated from each other and said insulating sheet having a thicknessof the order of 30 mils, and an electron multiplier so located in saidenvelope as to receive the secondary electrons which are released fromsaid mosaic when scanned by said beam, the beam current being of theorder of from micro-ampere to micro-ampere.

6. The invention according to claim 5 characterized in that saidphoto-electric elements are microscopic photo-sensitized silverglobules.

7. A cathode ray picture transmitting tube comprising an evacuatedenvelope having therein an electron gun for producing an electron beam,a photo-electric screen positioned to be scanned by said electron beam,said screen comprising a sheet of insulating material having on thefront side thereof facing said gun a mosaic of photoelectric elementsand having no back-plate, said elements being insulated from each other,and an electron multiplier so located in said envelope as to receive thesecondary electrons which are released from said mosaic when scanned bysaid beam, the beam current having substantially the minimum value forcausing said elements to reach equilibrium potential.

8. A cathode ray picture transmitting tube comprising an evacuatedenvelope having therein an electron gun for producing an electron beam,a photo-electric screen positioned to be scanned by said electron beam,said screen comprising a sheet of insulating material having on thefront side thereof facing said gun a mosaic of photo-electric elementsand having on the back side thereof a back-plate, said elements beinginsulated from each other and said insulating sheet being at least 10mils thick, and an electron multiplier so located in said envelope as toreceive the secondary electrons which are released from said mosaic whenscanned by said beam, the beam current having substantially the minimumvalue for causing said elements to reach equilibrium potential.

9. A picture transmitting tube comprising an electron gun for producinga beam current, a mosaic of elements positioned to be scanned by saidbeam, and an electron multiplier positioned to receive electrons fromsaid mosaic and to supply them in amplified form to an output circuit,the capacity of the effective picture elements of the mosaic being atleast as small as when the mosaic has a back plate spaced from saidelements by an insulating sheet 10 mils thick, and the ratio of the beamcurrent to said capacity being such that the beam current hassubstantially the minimum value for causing said elements to reachequilibrium.

GEORGE A. MORTON.

